The present invention relates to an active shaft ground system having a voltage sensing brush and a current feedback brush in sliding contact with a rotating shaft.
In rotating machines, for example large turbine generators, it is not feasible to establish a reliable passive ground connection to the shaft. In the absence of a suitable ground connection, a potentially destructive voltage can build up in the rotating shaft, which voltage can produce localized heating, possibly resulting in failure of shaft bearings. This appears to be due to the fact that the shaft, when rotating, acts as a high impedance electrostatic potential generator.
In order to prevent such voltage buildup, it is known to employ an active shaft ground system which includes a sensing brush contacting the shaft in order to sense the shaft voltage and a feedback brush which also contacts the shaft and is connected to apply a current having a value which neutralizes the electrostatic charge buildup on the shaft. Circuitry connected between the brushes acts to maintain the feedback current at the requisite level. The circuitry is arranged to vary the feedback current in response to shaft voltage variations.
While such active shaft ground systems have proven satisfactory in practice, there have been cases in which potentially harmful momentary shaft voltages have developed during the occurrence of feedback brush bounce, resulting in temporary loss of contact between the feedback brush and the shaft surface. During these times, the active shaft ground system is rendered ineffective and the shaft voltage is free to rise. Measurements performed on a large turbine generator have shown that the shaft voltage can rise by as much as 50 volts during a brush bounce intervals of 1.7 ms.
In existing systems, such brush bounce condition cannot be detected and it appears that repeated occurrence of this condition can eventually result in machine damage.
If this condition could be reliably detected, corrective measures, such as brush replacement or adjustment, could be taken before significant damage occurs.
Another condition which can occur on an intermittent basis during the operation of such machines is one in which the shaft rubs against a stationary machine part which is at ground potential. When this occurs, short circuit currents can flow between the shaft and ground. Since this condition causes the shaft voltage to fall to zero, the feedback current will correspondingly drop to a zero value. A rub condition cannot be easily corrected and normally requires a complete overhaul of the machine.
Since, however, brush bounce may be easily correctable, it is desirable to be able to distinguish between the two conditions.